U.S. patent number 4,084,437 [Application Number 05/716,750] was granted by the patent office on 1978-04-18 for thermocouple circuit.
This patent grant is currently assigned to Texas Instruments Incorporated. Invention is credited to Francis Finnegan.
United States Patent |
4,084,437 |
Finnegan |
April 18, 1978 |
Thermocouple circuit
Abstract
A thermocouple circuit has dissimilar conductors joined to form
a conventional temperature transducer located in a zone to be
monitored or controlled. The opposite ends of these conductors are
connected to similar metal leads in a capsule located in a stable
oven to form a reference junction. The oven also houses an
inexpensive integrated circuit operational amplifier which is
electrically connected to the similar metal leads to yield a high
gain output signal proportional to the weak thermal e.m.f. in the
dissimilar conductors. This signal is compared to a preset
reference voltage level and the difference signal converted to a
temperature correction signal which is in binary form. The oven,
the operational amplifier, and the binary signal producing
circuitry are all driven by a power source which has as its
reference or common potential an isolated voltage level tapped off
the shielded secondary winding of an otherwise conventional
transformer. The binary signal is de-isolated by providing an
optical coupler to other non-isolated equipment for operation of a
heater located in the zone.
Inventors: |
Finnegan; Francis (Wrentham,
MA) |
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
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Family
ID: |
24525096 |
Appl.
No.: |
05/716,750 |
Filed: |
August 23, 1976 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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629937 |
Nov 7, 1975 |
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427863 |
Apr 2, 1974 |
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Current U.S.
Class: |
219/209; 374/182;
374/E7.013; 374/E7.016 |
Current CPC
Class: |
G01K
7/021 (20130101); G01K 7/12 (20130101); G05D
23/1909 (20130101); G05D 23/22 (20130101) |
Current International
Class: |
G01K
7/12 (20060101); G01K 7/02 (20060101); G05D
23/20 (20060101); G05D 23/22 (20060101); G01K
007/12 () |
Field of
Search: |
;73/361,362.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ciarlante; Anthony V.
Attorney, Agent or Firm: Haug; John A. McAndrews; James
P.
Parent Case Text
This is a continuation of application Ser. No. 629,937, filed Nov.
7, 1975 now abandoned, which in turn is a continuation of prior
application Ser. No. 427,863 filed on Apr. 2, 1974, now abandoned.
Claims
I claim:
1. A thermocouple circuit comprising
(a) a pair of dissimilar conductors adapted to extend into a zone,
the temperature of which is to be monitored, the pair of conductors
having ends and opposite ends,
(b) a transducer defined by the ends of said dissimilar
conductors,
(c) an operational amplifier coupled to the opposite ends of said
conductors,
(d) a disk like capsule of thermally conductive material, said
opposite ends of said conductors embedded in said capsule disposed
adjacent to the operational amplifier in heat transfer relation
thereto, and
(e) a self regulating oven comprising a heating element formed of a
material having a relatively steeply sloped positive temperature
coefficient (PTC) of resistivity, the oven defining a chamber
therein, the heating element adapted to heat the chamber to a
predetermined temperature, the capsule and amplifier disposed in
the chamber.
2. A thermocouple circuit as set forth in claim 1 wherein said
heating element comprises an annular heating element circumscribing
said chamber.
3. A thermocouple circuit as set forth in claim 2 wherein said
annular PTC element comprises a polycrystalline semi-conductor
material, and a source of electrical power connected to said PTC
element in an electrical series circuit.
4. A thermocouple circuit as set forth in claim 3 wherein said oven
means further includes a thermally conductive cup-shaped member
inside said annular PTC element, said operational amplifier located
inside said cup-shaped member, and said capsule in which said
opposite ends of said thermocouple conductors are encased, lead
wires coupling said operational amplifier to said opposite ends of
said conductors, said lead wires having ends also encased in said
capsule, and said capsule also located in said cup-shaped oven
member.
5. A thermocouple circuit as set forth in claim 4 and further
including a source of symmetrical D.C. power for said operational
amplifier and including a reference "zero" voltage, and a source of
electrical power for said self regulating oven also referenced to
said "zero" voltage.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
The disclosure herein is also included in patent application Ser.
No. 427,942, filed on Dec. 26, 1973, in the name of Norman O.
Fonteneau and assigned to the assignee hereof.
SUMMARY OF INVENTION
This invention relates generally to thermocouple circuits, and
deals more particularly with a circuit for amplifying the
relatively weak thermal e.m.f. generated in the loop formed by the
dissimilar thermocouple conductors.
The thermal e.m.f. is initially amplified by a monolithic
integrated circuit operational amplifier as described in the above
mentioned copending patent application, and an isolated reference
potential is provided both for the operational amplifier, and also
for a differential amplifier tied to a sensitive potentiometer and
capable of providing a "difference" signal indicative of the
difference between a preset temperature and the actual temperature
sensed by the thermocouple.
Basically, a source of A.C. power is fed to a transformer which has
its secondary winding shielded and center-tapped to provide the
isolated reference voltage. A diode bridge type of rectifier is
driven by the secondary winding, and is also referenced to the same
potential to provide a stable source of symmetrical D.C. power.
This D.C. power drives both the operational amplifier and the
differential amplifier, and also suitable readout equipment. The
system also provides for temperature control, as well as
temperature monitoring, and a binary signal for such control is
"de-isolated" for use in driving conventional heating components
through an optical coupling device.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic view of a temperature monitoring and
controlling system incorporating the present invention.
FIG. 2 is a vertical sectional view through a component oven, and
its contents, of the type depicted schematically in FIG. 1.
FIG. 3 is a horizontal sectional view through the capsule shown in
the FIG. 2 oven.
DETAILED DESCRIPTION
Turning now to the drawings in greater detail, FIG. 1 shows a
conventional thermocouple junction or transducer 10 formed by the
connection between adjacent end portions of two dissimilar
conductors 12 and 14. The opposite ends of these conductors 12 and
14 (indicated at 12a and 14a respectively) are electrically
connected to similar metal leads 13 and 15 respectively in
accordance with conventional thermocouple practice to form a
reference junction.
FIGS. 2 and 3 show the physical connections 12a and 14a between the
thermocouple conductors 12 and 14, and their associated similar
non-critical lead out wires 13 and 15. As shown in FIG. 3, the
junctions 12a and 14a are defined in a capsule 17 which is
generally cylindrical in shape, and which is adapted to be mounted
in a component oven best shown in FIG. 2. The oven 60 will be
described in greater detail hereinbelow and is indicated
schematically in FIG. 1. Briefly, said oven 60 comprises a
conventional element of the combination herein, being of the
general type shown and described in U.S. Pat. No. 3,414,705 issued
Dec. 3, 1968 and assigned to the assignee herein. An oven of this
general type is adapted to be driven from either a source of A.C.
or D.C. power, and in accordance with the present invention is
preferably driven by the secondary winding of a transformer 19. The
transformer preferably is of the shielded type, and it is a feature
of the present invention that the secondary winding has a center
tap 16 which provides a convenient isolated common or reference
potential not only for the secondary winding which drives the
component oven of FIG. 2, but also for other components of the
system to be described.
A conventional source of A.C. power is indicated generally at 52 in
FIG. 1, and provides a source of A.C. electrical energy to the
primary winding of the transformer 19. The shielded secondary
winding of the transformer 19 provides A.C. electrical energy to a
unit 18, and more particularly to a rectifier of the diode bridge
type 18a. The unit 18 also includes a conventional regulator 18b,
and the unit 18 provides a symmetrical D.C. voltage output
represented generally by +V and -V both of which outputs are
referenced to the isolated common potential 16 defined by the
center tap on the shielded secondary winding of the transformer
19.
The thermocouple reference junction 12a is shown in FIG. 1 to be
electrically connected to the isolated common potential 16, and the
thermocouple reference junction 14a provides the positive input to
an inexpensive operational amplifier OA of integrated circuit
construction and such operational amplifier OA is driven by the
symmetrical source of D.C. power provided by the unit 18 with a
feedback loop 20 for decreasing the gain of the amplifier OA and
for passing signal frequencies within a predetermined bandwith. By
way of example, an operational amplifier of this general type, sold
by the assignee herein under Model No. 52741 is capable of
amplifying a relatively weak signal on the order of 25 microvolts
per degree Fahrenheit to the more readily useable level of 5
millivolts per degree Fahrenheit. It is characteristic of this type
of operational amplifiers generally, however, that the output
thereof is not reliable from the point of view of temperature drift
especially for input signals of this magnitude. Therefore, an
operational amplifier of this general type is not usually suitable
for amplifying relatively weak signals, such as the 25 microvolts
per degree signals in a thermoelectric loop without temperature
stabilization. A typical inexpensive integrated circuit operational
amplifier may be expected to have a thermal drift on the same order
of magnitude as the signal strength itself in a thermocouple
sensing circuit. Therefore, the inexpensive high gain amplifier OA
is stabilized to overcome its inherent disadvantages by stabilizing
its environment in the system described herein.
An important feature of the present disclosure is that the
operational amplifier OA is provided in the same stable oven 60
where the thermocouple reference junctions, 12a and 14a, are
located as described above. Thus, and as shown schematically in
FIG. 1, the amplifier OA together with the thermocouple reference
junctions, 12a and 14a, are located in the stable oven 60 to be
described in greater detail with specific reference to FIG. 2.
Still with reference to FIG. 1, the output of operational amplifier
OA in line 24 is fed to a second differential D.C. amplifier 26
which second amplifier 26 is also operated closed loop as indicated
generally by the feedback loop 28. The second amplifier 26 is also
referenced to the isolated common potential level as indicated
generally at 16. The output of the operational amplifier OA is
actually fed to the negative terminal of the differential amplifier
26, and a reference input to the positive terminal of the
differential amplifier 26 is preferably provided in the form of a
manually presetable voltage, determined by means of a thumb wheel
switch 32. The thumb wheel switch 32 embodies a sensitive
potentiometer by means of which a set temperature can be provided
as a reference from which the differential amplifier 26 operates.
This potentiometer embodied by the thumb wheel switches (not shown)
is referenced to the D.C. output voltage of the rectifier 18 and
the differential amplifier 26, like the operational amplifier OA,
is driven from this same source of power, and is referenced to the
isolated common potential 16 referred to hereinabove. It should
perhaps be noted that the output in line 24 might be used to drive
a visual readout device (not shown) for a direct indication of
temperature. However, the differential amplifier 26 and thumb wheel
switch 32 are provided to preset a desired temperature, actually to
preset a voltage input to the differential amplifier 26, so that
this amplifier 26 can compare the preset value with the actual
value from the operational amplifier OA to produce a difference
signal in line 27. This difference signal from line 27 is adapted
to drive a meter M, which meter is also referenced to the isolated
common potential 16.
The isolated circuitry of FIG. 1 further includes a potentiometer
34 to provide a reference voltage for a pulse width modulating
amplifier 36, which reference voltage is directly related to the
setting of the thumb wheel switch 32 and the output of operational
amplifier OA line 24. A pulse generator 38 is adapted to produce a
"saw tooth" output at a frequency set by the potentiometer 46, and
with a gradient or voltage-time slope dictated by the setting of
still another potentiometer 44. The potentiometer 44 comprises a
proportioning band potentiometer and provides a positive input to
pulse width modulating (PWM) differential amplifier 36. The output
of said (PWM) amplifier 36 comprises a rectangular waveform, the
width of each such rectangular pulse being determined by the
reference voltage input to the negative terminal of the
differential amplifier 36, and hence to the voltage difference
between the thumb wheel setting of switch 32, and the voltage level
in the output line 24 associated with the operational amplifier OA.
Both the differential amplifier 36 and the ramp pulse generator 38
are driven by the source of D.C. power 18, and are tied to the
isolated reference potential 16 referred to above. The differential
amplifier 36 is operated with feedback as indicated generally at
35, and its output in line 37 comprises a binary coded signal well
suited for use in driving an optical coupling device 40 as a result
of its simplified "on/off" vs. time signal. A light emitting diode
40a is energized when the "on" signal is present in line 37, and
the isolated common potential 16 is necessarily used at this side
of the optical coupling device 40. A photosensitive transistor 40b
conducts to provide a "de-isolated" control signal in the line 48
when the diode 40a is so energized.
Still with reference to FIG. 1, an electrically driven heater 50 is
provided in the zone, generally proximate to the thermocouple
transducer 10, and is adapted to be operated in response to the
magnitude of the difference signal from line 27. More particularly,
the electric heater 50 is adapted to be selectively driven from the
source of A.C. power 52 through a triac 54, the triac being
triggered from an A.C. power amplifier 42 whenever a de-isolated
control signal is present in line 48. The A.C. power amplifier 42,
and its associated trigger to the triac 54, as well as its
associated input from the line 48, are adapted to be referenced to
any convenient ground potential as indicated generally at 46, such
ground potential need not bear any electrical relationship to the
isolated common potential 16 described above. An optical coupling
device 40 is provided for de-isolating a binary signal, produced
from the analog signal in line 27, in order to allow the signal in
line 48 to be useable in equipment such as amplifier 42, which is
grounded to any convenient ground source, that is to a ground
potential not related to the isolated common potential 16 defined
at the center tap of the secondary winding of the transformer 19 as
described above. It will be apparent that the heater 50 can be
caused to operate for a portion of each cycle, as determined by
this frequency set at potentiometer 46 and as dictated by the
voltage level of the difference signal in line 27. As a result of
judicious selection of the settings for potentiometers 44 and 46 as
well as the potentiometer 34, it is possible to fit the system of
FIG. 1 to a particular environment and to optimize the cyclical
operation of the heater in a manner similar to a conventional time
proportional controller for a conventional temperature regulating
system.
Considering next the component oven shown in FIG. 2, the reader is
referred to the issued U.S. Pat. No. 3,414,705, referred to
previously, for a detailed description of a stable oven suitable
for receiving the operational amplifier OA and associated
thermocouple reference junctions 12a and 14a as described above.
However, and by way of brief summary, FIG. 2 shows such an oven as
comprising a cup-shaped aluminum holder 62, which holder has a
generally U-shaped cross section as seen in FIG. 2. The base
portion of the holder has openings for receiving the thermocouple
conductors 12 and 14 as well as their associated "non-critical"
lead out wires 13 and 15. The side walls of the cup-shaped holder
62 are surrounded by an annular ring 64 of PTC material. Such a
material is a semi-conductive one which displays a relatively steep
positive sloped resistivity-temperature curve or plot, and can be
used as a heat generating element which is self regulating as
described in the above mentioned patent. Two electrical lead out
wires 66 and 68 are provided for energizing the PTC element 64, and
these are preferably connected to the secondary winding of the
transformer 19 so that the electrical energy used to operate the
oven of FIG. 2 is necessarily referenced to the isolated source of
common potential 16.
The cup-shaped holder 62 may be anodized on its outer surface in
order to improve its electrical insulation properties without
sacrificing the desirable electrical conduction of aluminum. In the
alternative, the same result can be achieved by use of a suitable
thermoplastic resin coating in this area. A tubular casing 70
fabricated from nylon or other thermally and electrically
insulating material is provided outside the ring shaped PTC element
64, and outside the lead wires 66, 68 associated therewith. This
insulating casing 70 has a radially inwardly oriented flange
adapted to overlie the radially outwardly projecting flange of the
cup-shaped holder 62. The capsule 17, as described herein above
with reference to FIG. 3, is adapted to fit within the lower
portion of the cup-shaped holder 62 and the operational amplifier
OA is fabricated to fit within the remaining portion of the
cup-shaped holder, and to have its characteristic five lead out
wires projecting upwardly through the opening defined by the
flanged casing member 70. A cover member 72, also fabricated from
an insulating material, is fitted around the casing 70 and may be
formed of the same plastic thermally and electrically insulating
material, such as nylon. A cover 73 closes the top of the oven, and
a disc 74 is provided at the lower end of the casing 70. Suitable
openings are provided in both the disc 74 and cover 73. The disc 74
has openings for receiving the lines 66 and 68 associated with the
heating element 64, and also for receiving the thermocouple leads
12 and 14 and their associated non-critical leads 13 and 15. The
cover 73 has similar openings for the five leads of the amplifier
OA. The operational amplifier OA and the capsule 17 may be held in
the cup-shaped holder 62 by means of a potting compound 74 having
good thermal conductive properties. On the other hand, a thermally
insulating potting compound is provided in the voids defined
outside of the cup-shaped holder 62 and inside the casing 70 to
further improve the efficiency of the FIG. 2 oven. As a result of
this oven construction it will be apparent that the operational
amplifier OA is located in close thermal relation to the PTC
heating element 64 resulting in very efficient control of its
temperature, and of the temperature of the junctions 12a and 14a in
the capsule 17. Such an oven is available commercially from the
assignee herein and is sold under their KLIXON trademark as Model
No. 4ST1. Such an oven is adapted to control the temperature within
the cup-shaped holder at approximately 115.degree. C plus or minus
5.degree. C when operated from a 24 volt A.C. source as described
above. More particularly, the temperature shift caused by voltage
variation to the oven is only on the order of 4/10ths degree
centigrade C per volt change. When the oven is given three minutes
within which to warm up, the voltage gain of the operational
amplifier is not only held linear, but is also quite thermally
stable, and hence quite satisfactory when used in a circuit of the
type described hereinabove.
* * * * *